6.16.0 release notes

Yang Validation of Draft Submissions
This release adds Yang validation of submitted drafts which contain
Yang code, through a new plug-in architecture which makes it easy to
addo other submission-time checkers and validators in the future.
It also contains a few unrelated fixes and tweaks, as follows:
* Fixed pyflakes complaints introduced with pyflakes 1.1.0
* Refactored draft submission checks so that new checkers can be slotted
in through a configuration in settings.py. Refactored the calling of
idnits to use the new API, and added a pyang validation check.
* Added new entries to requirements.txt: pyang, xym, and jsonfield. Sorted
the list.
* Merged in [10880] from rjsparks@nostrum.com:
Only show the 'Upload new revision' button when the view will actually
let you upload a new revision.
* Merged in [10862] from rjsparks@nostrum.com:
Group concluded working groups by area. Fixes #1670.
* Merged in [10839] from housley@vigilsec.com:
Added test for proper eneration of the approval message with and without
an RFC Editor Note. Tweaked RFC Editor note display template.
-- Henrik Levkowetz <henrik@levkowetz.com> 05 Mar 2016 15:01:38 +0000